Danielle M. Frechette

Sea Lamprey ( Petromyzon marinus), a native species in rivers connecting to the Atlantic Ocean in Maine (USA), have been in steep abundance declines due to fragmentation of river habitats from impoundment. Recent improvements in connectivity in the Penobscot River have increased access to upstream habitats for the species. We used environmental DNA and radiotelemetry to assess the current dam passage and habitat use by adult Sea Lamprey in the Penobscot River. We then compared these results with previous studies and historical data to quantify changes in Sea Lamprey distribution through time. Our results show that the distribution of Sea Lamprey decreased by 84% between the 1800s and 2007. Restoration efforts have increased the species’ distribution by 45% between 2007 and 2023, but the species remains limited to only 23% of its historical range. Dams continue to prevent the upstream expansion of Sea Lamprey. Many dams in the Penobscot River lack fish bypasses, and passage performance is highly variable among dams with bypasses (0%–96% overall passage success between 2020 and 2023). Improved passage at dams remains an important restoration action for these native fish.

The Atlantic salmon Salmo salar L. is an endangered species in the United States, but post‐spawn downstream movements remain poorly understood. We conducted a 2‐year acoustic telemetry study to characterize downstream movements and to quantify apparent mortality and downstream passage of post‐spawn adults (kelts) in the Penobscot River, Maine, USA (N = 112). The majority of tagged S. salar kelts (54%) exhibited a rapid movement seaward post‐release instead of overwintering in the river. Salmo salar kelts that overwintered exhibited two patterns, distinguished by whether a dam was present or not. We found no relationship between post‐release movement pattern, sex, body condition index or release year. Estimated apparent mortality rates were high (83%), with a greater probability of mortality occurring near the dams and release site. Apparent mortality rate did not differ by sex, body condition or year, but it was greater for individuals that moved out directly post‐release and less for those that overwintered. While our results may suggest that overwintering in the river is a favourable pattern for survival, these results may be related to the closures of the designated downstream passages at the first two dams encountered by S. salar kelts. In fact, 55% of S. salar kelts passed a dam when the designated passages were closed. Altogether, these results may demonstrate the complex nature of downstream passage for S. salar kelts, particularly when having to navigate multiple dams.

As rivers continue to warm due to climate change, there is an urgent need for coordinated approaches to enhance the thermal resiliency of riverscapes. Cold‐water refuges (CWRs) serve as critical habitats for temperature‐sensitive aquatic species, yet efforts to enhance or create these refuges often lack a unified framework, risking a fragmented and ineffective approach. To address this challenge, we convened a workshop bringing together experts, practitioners, and stakeholders to develop best practices for CWR enhancement. This manuscript synthesizes the workshop outcomes, presenting a flowchart of overarching conceptual approaches to CWR enhancement, emphasizing factors such as refuge density and type. Case study summaries highlight examples across different CWR types, providing practical insights into site‐specific challenges and solutions. Key recommendations include prioritizing CWRs for maximum ecological benefit, integrating thermal refuge enhancement into broader riverscape management plans, and adopting a catchment‐based perspective to address upstream influences. We also identify critical knowledge gaps, such as the need for improved methods to assess CWR usage, including tools like drones and thermal infrared imaging. These gaps underscore the importance of continued innovation and collaboration to refine best practices. By providing a clear, actionable framework, this work aims to guide practitioners toward more effective, coordinated efforts to safeguard aquatic ecosystems under a warming climate.

Objective Sea Lamprey Petromyzon marinus provide important ecological services within their native range, such as nutrient cycling, and can also act as a prey source for other species. Adult Sea Lamprey must access freshwater rivers to spawn, and because of this they are susceptible to changes in river connectivity. Human-made structures, such as dams, can exclude them from usable habitat. Sea Lamprey dam passage has not been extensively studied in Maine, despite Maine being within the native range of this species. The goals of this study were to evaluate upstream passage efficiency at the Milford Dam on the Penobscot River, Maine, and to provide comprehensive information about adult Sea Lamprey passage at five other dams throughout the Penobscot River watershed. Methods In 2020–2021 we captured and tagged 150 Sea Lamprey at the Milford Dam, the lowest dam in the Penobscot River, Maine, and displaced them downstream to assess passage efficiency at this dam and five upstream dams. In 2020, 50 Sea Lamprey were released on the east shore of the river downstream of Milford Dam; in 2021, the east shore release was repeated with an additional 50 fish and another 50 fish were released on the west shore. Result Between 70–82% of Sea Lamprey were observed passing Milford Dam again after mean delay times of 9–11 days. The release location did not affect dam passage success or the amount of time that was required to locate and use the passage structures. Sea Lampreys from both release groups were equally likely to approach the entrance to the fishway upon returning to Milford Dam, despite the fishway being located against the eastern shore of the river. However, high flows shortly after release may have resulted in higher attraction to the fishway in 2020. Passage success at dams upstream of Milford was highly variable. All Sea Lamprey were able to successfully navigate past West Enfield Dam (100% passage, n = 63), whereas Brownsmill Dam apparently acted as a complete barrier to further migration (0% passage, n = 7). Fish from all years and release groups together had a median upstream migration distance of 38.8 km after fish had passed Milford Dam, and a maximum observed upstream travel distance of approximately 100 km, indicating that most tagged Sea Lamprey ended their migration in the vicinity of a dam. Conclusion The results of this study indicate that Sea Lamprey have high passage efficiency at the Milford Dam and highlight areas within the Penobscot River basin—such as the Brownsmill Dam—where passage facilities are currently inadequate for Sea Lamprey.

Human activities and climate change threaten coldwater organisms in freshwater ecosystems by causing rivers and streams to warm, increasing the intensity and frequency of warm temperature events, and reducing thermal heterogeneity. Cold‐water refuges are discrete patches of relatively cool water that are used by coldwater organisms for thermal relief and short‐term survival. Globally, cohesive management approaches are needed that consider interlinked physical, biological, and social factors of cold‐water refuges. We review current understanding of cold‐water refuges, identify gaps between science and management, and evaluate policies aimed at protecting thermally sensitive species. Existing policies include designating cold‐water habitats, restricting fishing during warm periods, and implementing threshold temperature standards or guidelines. However, these policies are rare and uncoordinated across spatial scales and often do not consider input from Indigenous peoples. We propose that cold‐water refuges be managed as distinct operational landscape units, which provide a social and ecological context that is relevant at the watershed scale. These operational landscape units provide the foundation for an integrated framework that links science and management by (1) mapping and characterizing cold‐water refuges to prioritize management and conservation actions, (2) leveraging existing and new policies, (3) improving coordination across jurisdictions, and (4) implementing adaptive management practices across scales. Our findings show that while there are many opportunities for scientific advancement, the current state of the sciences is sufficient to inform policy and management. Our proposed framework provides a path forward for managing and protecting cold‐water refuges using existing and new policies to protect coldwater organisms in the face of global change.

Climate change and human activities have dramatically affected all ecosystems inhabited by Atlantic salmon, causing drastic population declines. Change in river temperature dynamics (e.g. daily variability, frequency, and duration of summer maximum, warmer thermal regimes) is of special concern as it impacts growth rates, reproductive success, prey abundance and phenology, timing of migration, and ultimately survival. The Atlantic Salmon Research Joint Venture held a workshop to address the effects of climate change on freshwater habitats of Atlantic salmon and identify research gaps and priorities. Here we summarize the state of the science for three key themes identified by workshop participants: (1) Effects of climate change on in-river habitat conditions, (2) Physiological and behavioral responses of salmon to temperature, and (3) Population-level responses of salmon to climate change. The group highlighted the crucial importance of understanding and monitoring the links between river temperature dynamics and physiological requirements of Atlantic salmon across different life stages and habitat conditions, with a focus on freshwater life stages. Climate change will undoubtedly continue to affect instream habitats across all seasons and render challenging conditions for all freshwater Atlantic salmon life stages. Hence, we call for urgent interdisciplinary collaborations and partnerships among scientists and managers to address the pressing research gaps that require large-scale data integration across life cycle stages and ecosystems. More collaboration between scientists, managers, and interest groups is needed to ensure that fundamental science directly addresses the knowledge-action gap to enhance evidence-based decision-making and conservation. Climate change and anthropogenic activities are affecting Atlantic salmon habitat characteristics, leading to physiological and behavioral changes that determine both the individual and population level potential for adaptability. Although climate change affects all aspects of the Atlantic salmon life cycle and habitats across the watersheds-ocean continuum, this workshop focused on changes in thermal and hydrological river regimes.

The Gulf of Maine Distinct Population Segment of Atlantic Salmon Salmo salar is listed under the U.S. Endangered Species Act, with dams cited as a key threat to the species. Safe, timely, and efficient passage of emigrating smolts is critical for advancing recovery goals. However, climate change and the earlier (and more variable) onset of smolt migration have challenged the effectiveness of measures designed to protect smolts. Therefore, we used data from four long-term smolt trapping sites in Maine to characterize run duration, identify a standardized smolt run, and develop a predictive model for the initiation of smolt emigration for natural- and hatchery-origin smolts. We combined these data into a basinwide deterministic model that projects the movement of smolts from their point of origin to estuary entry, which we used to (1) evaluate duration and temperature triggers for supplemental spill periods used at hydroelectric dams to protect smolts and (2) investigate how knowledge of smolt emigration dynamics can guide protective measures in rivers where management actions are being developed. Timing of run initiation varied by over 14 d; however, mean air temperature for the first quarter of the year explained over 60% of the variance in the onset of emigration. Initiation temperature was linked with higher-elevation rearing areas where smolts originated and not with downstream dams where temperature was monitored. The shape of the smolt wave varied considerably; thus, our standardized smolt wave captured average dynamics but not the specific shape. Overall, 14-d spill windows are too short. To account for the variable timing and shape of the run, a 54–72-d protection window would be necessary to protect the earliest and latest emigrants in the absence of physical structures (e.g., turbine screens) and preserve the adaptive variation required for populations to respond to climate-driven changes in temperature and hydrology.

Allowing reproductive individuals to colonize novel habitat or recolonize previously occupied habitat is increasingly being considered as a tool for recovery of depleted populations of anadromous salmon. Successful application of these techniques requires thorough understanding of how adults use the riverscape during colonization to ensure that programs achieve desired outcomes. We examined the movements and habitat use of adult Atlantic SalmonSalmo salarduring colonization of novel habitat in an eastern Canadian river using a novel combination of acoustic telemetry, remote sensing, ground surveys, and continuous records of river temperature and discharge. Females moved less than males, regardless of river temperature or discharge, whereas males engaged in more extensive movements except at elevated temperature and discharge. Probability of movement was lower during the summer, coincident with individuals holding in pools during high‐heat/low‐discharge events. River temperature, discharge, and day of year were influential in predicting whether salmon held in pools, and size was the most important physical characteristic identifying “suitable” holding pools. Observed movement patterns may reflect different evolutionary strategies employed by each sex to maximize reproductive fitness. Because spawning behavior is highly conserved within salmonids, these findings may (1) provide a generalized picture of how Atlantic Salmon use space during colonization of unoccupied habitat and (2) be used to optimize future reintroduction and assisted migration programs.

Transport of salmonids allows upstream migrating adults to bypass barriers to migration, for example hydroelectric dams or adverse habitats. Downstream movement (“fallback”) after transport is common and removes individuals from spawning populations, with possible detrimental consequences for population productivity. Time‐to‐event analysis was used to determine effects of fish characteristics, transport conditions and environmental variables on fallback by adult Atlantic salmon transported to an inaccessible river reach as a population enhancement strategy on the Rivière Sainte‐Marguerite Nord‐Est (Québec, Canada). Of 68 salmon transported, 19 exhibited post‐transport fallback within seven days of transport. Fork length (FL) was the only factor that increased risk of post‐transport fallback; there was a 5% increase in the proportion of large salmon (≥780 mm FL) exhibiting post‐transport fallback relative to individuals of median size (750 mm FL). Although the present study was limited by small sample size, the findings will help inform design of transport programmes in freshwater systems.